Thermoplastic polyester resins, especially polyethylene terephthalate and polybutylene terephthalate, are used in various fields with or without reinforcement with glass fibers or the like, and serve as engineering plastics having excellent mechanical and electrical properties and also having good chemical resistance and heat resistance. However, they are defective in that they are combustible. Therefore, from the industrial viewpoint, it is important to render these thermoplastic resins flame-retardant.
There are known various methods for rendering thermoplastic polyester resins flame-retardant. Ordinarily, a flame retardant capable of imparting a flame retardance is incorporated alone or in combination with a flame retardant assistant to obtain a flame-retarding effect. However, when known flame retardants are incorporated into polyester resins for exterior articles, significant discoloration is caused in the resulting exterior articles by the action of solar rays and their colors become incongruous with colors of other exterior articles, with the result that the value of the exterior articles is reduced. In case of articles other than such exterior articles, if molded articles of thermoplastic resins containing conventional flame retardants are used in a high-temperature atmosphere, the flame retardants bleed out on the surfaces of the molded articles to cause an undesirable whitening phenomenon. Accordingly, these flame retardants are not suitable for thermoplastic polyester resins for use in production of exterior articles and electrical parts to which a high voltage is applied.